1. Field of the Invention
The present invention relates to an improved method of processing of phosphate matrix slurries. More particularly, the present invention relates to a method of reducing the viscosity of phosphate matrix slurries, and improving the consolidation characteristics of phosphatic clays remaining in the water suspension, after the phosphate ore beneficiation process is completed, by drying the phosphate ore utilized to form the slurries.
2. Discussion of the Prior Art
The problems associated with handling of phosphate-containing ores in phosphoric acid manufacturing processes are well recognized. One of the principal problems associated with handling of phosphate matrix (low grade, poor quality clay-bearing phosphate rock) is the viscosity of phosphate matrix slurries. The phosphate matrix is normally slurried in order to facilitate transport of the phosphate-containing ore from a mining site through a pipeline or conduit to a beneficiation facility. In order to promote the efficiency of this transport process, it is highly desirable to maintain as high a slurry concentration as possible without causing extraordinarily high viscosities or excessive pump wear. Accordingly, it is highly desirable to reduce the viscosity of the phosphate matrix slurries so that the energy requirements of the pumping process can be reduced and the productivity of the process can be increased.
Various methods and additives for reducing the viscosity of phosphate matrix slurries have been proposed in the prior art. For example, U.S. Pat. No. 3,035,867 to Corbett teaches the use of additives selected from the group consisting of free coal acids, alkali metal and ammonium salts of free coal acids, and mixtures of these compounds in order to reduce the viscosity of slurries containing phosphate rock. U.S. Pat. No. 4,042,666 to Rice et al teaches the addition of an additive selected from the group consisting of aliphatic hydroxy acids containing 2-6 carbon atoms, water-soluble inorganic salts of the aliphatic hydroxy acids, and blends of the aliphatic hydroxy acids or the aliphatic hydroxy acid salts with a strong base in a process for treating clay-containing phosphate rock slurries in order to reduce the viscosity of these slurries. Additional examples of prior art additives and processes for reducing the viscosity of phosphate matrix slurries by chemical means are disclosed in U.S. Pat. No. 4,177,243 to Schwartz et al; U.S. Pat. No. 4,220,630 to Ore'; U.S. Pat. No. 4,374,817 to Lehman et al; and U.S. Pat. No. 4,402,923 to Lang.
The above-cited prior art references generally teach that viscosity reducing agents may be added at various points during processing of the phosphate matrix. For example, U.S. Pat. No. 4,177,243 to Schwartz et al teaches that the viscosity reducing agents described therein may be added during grinding of the phosphate matrix, or may be added to a slurry of the ground phosphate matrix. U.S. Pat. No. 3,035,867 to Corbett teaches the addition of the viscosity reducing agents by means of either dry mixing the agent with the phosphate matrix or the addition thereof to a slurry containing the ground phosphate matrix. U.S. Pat. No. 4,374,817 to Lehman et al teaches that the viscosity reducing agents described therein can be added at any stage during the preparation of the phosphate matrix slurries, but are preferably added during wet grinding of the phosphate matrix. U.S. Pat. No. 4,042,666 to Rice et al teaches that a chemical treatment to prevent swelling of residual clays and thereby reduce the viscosity of phosphate matrix slurries may be employed at any stage during processing of the phosphate matrix.
Another problem associated with the recovery of phosphate values from the phosphate matrix is the recovery of water used in slurrying the matrix. The matrix, usually mined by surface mining methods, is usually comprised of clay, silica sand and phosphate. Majority of the clay particles are in the size range of less than one (1) micron (.mu.). After the matrix is mined, it is conventionally combined with water, without any pretreatment of the matrix, to form a slurry, which is subjected to washing, screening, agglomeration and classification before it is transported to the phosphate beneficiation operation, e.g., agglomeration flotation or froth flotation. During the washing operations, extremely finely divided material, originally in the matrix, remains in the aqueous suspension. This aqueous suspension is commonly called "slime" or "phosphatic slime", and it is produced in large quantities during the phosphate ore beneficiation process. The slimes usually amount to about 20-40% of the phosphate ore mined. When it is considered that the slimes are formed as aqueous suspensions containing 1-5% solids, it becomes apparent that enormous quantities of water are used in the phosphate matrix processing.
The disposal of vast amounts of the generated slimes and the recovery of residual phosphate values from slimes presents pressing problems for the phosphate industry.
Slimes present a problem because they retain substantial amounts of their original water, even after years of settling. Not only does this result in an area having no bearing strength, but it also results in the waste of increasingly rare natural resources, water and land. Even after settling for many years, the slimes settle to only about 20% by weight of solids and still are jelly-like in consistency. As a result, there are large areas near the phosphate producing areas of Florida that are virtually useless for any purpose.
The slimes are retained in dammed ponds which must be continuously inspected and repaired. Such dams may fail, thereby polluting the land, rivers and lakes. The pollution damage may be especially severe in the case of polluted waters because of the potentially lethal effects on fish and other life.
The industry has attempted over the years to eliminate the wasteful loss of phosphate values, water and land. For example, U.S. Pat. No. 4,194,969, discloses that the treatment of a phosphate ore matrix with an electrolyte solution containing metal ions, e.g., aluminum, calcium, iron and zinc, and anions, e.g., nitrate, chloride, sulfate and acetate hydroxide, facilitates more rapid slimes sedimentation. Other slimes formation modifying techniques are disclosed in U.S. Pat. Nos. 3,008,575 of Clawson et al, and 3,314,537 of Greene.
Every et al, U.S. Pat. No. 3,359,037 disclose a method of mining of phosphate matrix comprising contacting the matrix deposit with a mineral acid, e.g., sulfuric, phosphoric or nitric acids, to chemically attack the nonphosphatic material cementing the phosphatic particles.
However, none of the previous methods met with the success required in commercial operations.
Accordingly, it is a principal object of the present invention to provide an improved method of processing phosphate matrix slurries.
Additional objects of the present invention are to provide methods of reducing the viscosity of phosphate matrix slurries and improving the consolidation characteristics of slimes obtained as a by-product of the phosphate matrix beneficiation process.